Transmission or ΔE detectors of high purity silicon are commercially available and indeed up to a thickness of 1 mm. Occasionally, ΔE detectors up to 2 mm thick which are composed of high purity silicon can be obtained.
If ΔE detectors are to be made with greater thicknesses of 2 to 10 mm, pure silicon can no longer be used since that material is not available with the requisite degree of purity. Previously, lithium-compensated silicon [Si(Li)] has been used as the base material for this purpose. Such commercially available Si (Li) detectors have a small p+ contact which has a thickness of less than 1 μm. It is in the form of a surface barrier layer which can be fabricated by vapor deposition of a metal layer and indeed has especially been produced by vapor deposition of gold or by p+ contact made by boron implantation. In addition, these Si (Li) detectors have a 100 to 500 μm thick Li-diffused n contact. Within the 100 to 500 μm thick n contact layer a charged particle or photon will lose its energy without a reaction by the detector thereto. The region of the layer in which the detector does not react to the energy loss of the charged particle is termed the “dead zone” in the description which follows. The dead zone thus does not contribute to the determination of the energy of photons or charged particles.
In the publications IEEE Trans. Nucl. Sci. NS-25, No. 1(1978) 391; IEEE Trans. Nucl. Sci. NS-31, No. 1 (1984) 331; IEEE Trans. Nucl. Sci. NS-43, No. 3 (1996) 1505, method have been descried which seek to reduce the thickness of the Li diffused contact to 10 to 20 μm. There remains however a 10 to 20 μm thick layer and hence always a still relatively thick insensitive region or dead zone in the Si(Li) detector. Such a dead zone has an effect in practice as has already been remarked and is detrimental for a transmission detector.
Several authors have reported on ΔE detectors of Li compensated silicon in which the Li diffused n contact has been completely removed. Instead of the n contact, an ohmic contact is produced by vapor deposit of an aluminum layer as can be deduced from the publication Jpn. J. Appl. Phys. 33 (1994) 4115. These Si(Li) detectors, also of Li compensated silicon function well for a time after manufacture. However the same authors have reported on serious problems with the long-term stability of such detectors with ohmic contacts as can be deduced from the publication Jpn. J. Appl. Phys. 33 (1994) 4111 and Jpn. J. Appl. Phys. 34 (1995) 3065.
From the publication G. F. Knoll, “Radiation Detection and Measurement”, Chapter 13, John Wiley & Sons, New York 2000, it has become known that the Si (Li) detectors retain their properties for some time and as a rule for several months after manufacture. Then, however, there begins to occur a redistribution of the lithium within the compensated volume. Several lithium ions, which are combined to acceptor ions meander to the crystal defects or other impurities. The compensated silicon transforms itself into the p type. To reduce the problems, the manufacturer seeks at room temperature to always apply a bias voltage to the detector when the same is not in operation. In this manner the compensation loss can be made up by “after drifting”.
Such a treatment cannot be carried out with transmission detectors which are fabricated without Li diffusion contact since a p-layer develops again below the ohmic contact. As a consequence long-term stability problems arise.
Si(Li) detectors can be damaged by the radiation in the course of measurement. Such damaged Si(Li) detectors are regenerated by so-called “uredrifting” i.e. drift at temperatures between 50° C. and 100° C. as has been described by the publication M. Saskola and K. Nybo, Nucl. Instr. Meth. 44 (1966) 141. In this case because of redrifting the lattice defects (usually of the p-type) resulting from radiation are compensated by Li ions. With transmission detectors without Li diffused contacts, such a process is not possible since a p-layer forms beneath the ohmic contact. On this ground as well there are long-term stability problems.